Re: [Boost-users] [fusion] Why no multi{map,set} ?

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-08-13 20:53:17

AMDG

John C. Femiani wrote:
> I was just wondering why fusion (and mpl) don't include multi-maps or 
> multi-sets. Are they unnecessary at compile time? Are they too 
> trivial? Do they conceptually not work?

The easiest way to implement a multimap is in terms of an ordinary map.
In a fusion/mpl map the key is a type
and lookup uses overload resolution.  This means that there
is necessarily a 1-1 correspondence between keys and values.
The obvious way to implement a multimap is to make the
value a container.  Because of the function nature of
fusion/mpl  this would be like

template<class M, class K, class V>
struct multimap_insert_existing {
    typedef typename insert<typename erase_key<M, K>::type, pair<K, 
typename push_back<typename at<M, K>::type, V>::type> > type;
};

template<class M, class K, class V>
struct multimap_insert_new {
    typedef typename insert<M, pair<K, vector1<V> > > type;
};

template<class M, class T>
struct multimap_insert {
    typedef typename T::first key_type;
    typedef typename T::second value_type;
    typedef typename eval_if<has_key<M, key_type>, 
multimap_insert_existing<M, key_type_value_type>, multimap_insert_new<M, 
key_type, value_type> >::type type;
};

In Christ,
Steven Watanabe